1. Field of the Invention
The present invention generally relates to devices used to electromagnetically mark and locate obscured objects, and more particularly to a digging guard adapted to visually mark a buried object, such as a splice closure, which also provides a transponder or marker to enable the later location of the buried closure.
2. Description of the Related Art
Buried conduits are employed for supplying a wide variety of utilities, including pipelines for gas, water and sewage, and cables for telephone, optical fiber, power and television. It often becomes necessary to locate defective or damaged cables, pipes, etc., in order to repair or replace them or to quickly locate a buried, high-pressure gas valve. Conversely, it is important to know with as much accuracy as possible the approximate vicinity of such items in order to avoid disturbing them when digging or excavating for other purposes, such as emergency situations. Above-ground marking devices may be installed immediately after the conduit is buried, but they are often lost, stolen, or destroyed after a short period of use. Therefore, it is common to use underground marking devices or systems to enable the later location of a section or feature of an underground utility.
In the past, three different approaches have been used to indicate the presence of buried conduits, namely, warning tapes, trace wires, and electronic marker systems. A warning tape is simply a band of plastic which is placed above the conduit before burial. These tapes are used to alert the excavation team of the presence of the conduit before any damage thereto might occur. As the backhoe or other mechanical digger excavates the site, it will hopefully uproot a portion of the warning tape prior to contact with the conduit. The primary disadvantage of (non-metallic) warning tapes is that they cannot be detected by any surface instrumentation.
A single trace wire is sometimes buried with a utility line. The trace wire is used as a conductor for an AC signal which is applied to the wire at one accessible end, and then acts as an antenna and radiates an electromagnetic field above ground along its entire length. The electromagnetic field may be detected with an appropriate receiver, and the underground path of the line thereby determined. The earliest cable locators used a single sensor which detects a single null or peak (depending upon the orientation of the sensor) as the unit passes near the cable. Many later devices use two or more sensors that combine the signals to provide an indication of conductor proximity. The most common sensors are ferrite-core antennas, i.e., inductors. Although the conduit itself may act as a conductor (i.e., when steel pipe or copper wire cabling is used), most conduits are non-conductive and therefore require a trace wire. There are three significant disadvantages in the use of a trace wire. First of all, it is necessary to provide above ground access to the trace wire in order to couple the AC signal thereto. Secondly, if a break occurs in the wire (due to excavation, or natural causes such as corrosion, earth movement or burrowing animals), then the wire becomes useless. Finally, the trace wire is too thin to imprint a warning message thereon, precluding any visual warning. Additionally, a receiver cannot distinguish the trace wire from any other conductor in the vicinity. Marking of the path still does not identify specific components, like valves or splices.
Electronic marker systems for locating buried objects are known in the art, and generally consist of two types, namely, active and passive markers (transponders). Active markers require the use of a power supply which amplifies a signal source (usually an AC signal). The signal is radiated by the underground marker and detected by a receiver unit above ground. Passive markers, in contrast, have no power supply, but rather operate in a resonant mode, responsive to a transmitted electromagnetic field.
A passive marker is basically a wire coil and capacitor surrounded in a protective envelope, which is then buried adjacent to the cable, pipe, or other object to be located. The marker is self-contained, with no external, accessible connections. Passive markers are activated by radiating a signal into the ground in the area where the marker is expected to be found. The signal is emitted via an inductive coil held close to the surface (the transmitter portion of a transceiver). When the coil is directly over, or near, the passive marker (which is itself an inductive coil), the marker accepts energy within its bandpass and stores it, reaching a sustained amplitude during the transmission cycle. When the transmission cycle ends, the marker re-emits the energy at the marker""s resonant frequency with an exponentially decaying amplitude. A second coil within the transceiver unit acts as a receiving antenna which detects the re-radiated energy, alerting the locating technician with an audible tone or other indicator means. See generally, U.S. Pat. No. 5,045,368.
FIG. 1 illustrates several kinds of passive transponders for different applications. These include a small, near-surface marker 2 for locating a valve box, a medium size or mid-range marker 4 for locating a service drop (a loose coil of cabling deployed for future use), a full-range marker 6 for locating a more deeply buried conduit stub, and a so-called ball marker 8 for locating a conduit tee. The latter marker provides a spherical housing which supports the marker coil horizontally, regardless of the orientation of the housing (i.e., self-leveling), and is used for soil conditions which may result in significant shifting of the housing, such that the marker always provides a vertical location beacon (inductor axis). These markers simply xe2x80x9cfloatxe2x80x9d around the underground feature in the soil, and are subject to soil movement. They all have a relatively minimal profile.
Electronic markers, as well as warning tapes, are usually color-coded according to the particular type of utility line they mark. Specifically, gas line markers are yellow; telephone cable markers are orange; waste water tunnel markers are green; water pipe markers are blue; and power supply markers are red. Similarly, the passive marker is xe2x80x9ccodedxe2x80x9d by tuning the coil for a specific resonant frequency. Five distinct frequencies have been designated: 83.0 kHz for gas; 101.4 kHz for telephone; 121.6 kHz for sewage; 145.7 kHz for water; and 169.8 kHz for power. In this manner, a locating technician searching for, say, a gas line, cannot accidentally activate a telephone marker since his transmitter will only be sending out an 83 kHz signal, which is not within the bandwidth for a telephone marker tuned for 101.4 kHz. Of course, these frequencies have been designated by convention, and are not meant to be restrictive.
There are hybrid systems wherein, for example, a signal is applied to a buried conductor (cable or trace wire), and coupled through the conductor to one or more markers buried adjacent the conductor. Also, a marker can be used to couple one conductor to another, so that the test signal may be conveyed to the second conductor without a direct physical connection.
While several of the foregoing articles can be used to alert an excavation team that a buried object is nearby, there continue to be problems regarding both locating and damaging the cables or conduits. Even with correct placement procedures, movement can occur in a filled ditch or hole as a result of loose unpacked soil, freeze/thaw cycling, water erosion, and other causes. These conditions can cause a marker to become oriented at a different axis with respect to the vertical, which usually results in a mislocate of the obscured object. Self-leveling markers are available, but the self-leveling construction adds significant expense to the product. The self-leveling markers (such as the ball marker 8) also provide no expansive visual indication to the excavation team, i.e., if the backhoe or other mechanical digger excavates the site, then by the time the ball marker is seen, the backhoe blade is already dangerously near the marked object (the ball marker is only about the size of a grapefruit). It is possible that the backhoe would scoop up the ball marker in a load of dirt without even being noticed by the operator. While marker tape offers a more expansive visual indication, there is still no guarantee that the backhoe shovel will grab onto the marker tape and reveal it, e.g., if the backhoe blade is thrust at a glancing angle near the edge of the tape.
In light of the foregoing, it would be desirable to devise an improved article for more accurately locating a buried object. It would be further advantageous if the article could provide not only improved electronic locating, but also provide a strong visual indication of the proximity of the object which is easily detected by an excavator.
It is therefore one object of the present invention to provide an improved article for deploying an electronic transponder used to locate a buried or otherwise obscured object.
It is another object of the present invention to provide such an article that allows accurate orientation of the transponder.
It is yet another object of the present invention to provide such an article which is highly visible and easily detected during excavation, so as to avoid damage to the buried object that might otherwise occur while digging.
The foregoing objects are achieved in a buried closure guard, generally comprising an electronic transponder, and a generally flat, perforated carrier sheet constructed of a flexible polymer. In one embodiment, the transponder is generally flat and oriented parallel with the sheet, and is a passive transponder having a resonant LC circuit. The holes in the carrier sheet preferably have an effective diameter of at least about 1xc2xdxe2x80x3 (the holes can be elongate). The carrier sheet can also be color-coded to indicate a particular utility; e.g., telecommunications. By providing a webbed carrier, any protrusion of a digging tool (such as a backhoe bucket) will catch on one of these holes and pull at the carrier to make it further visible during excavation. The perforated construction of the sheet allows backfill material to settle and pass through the carrier without significantly shifting or bending the carrier. The carrier also makes it easy to manually deploy the transponder, i.e., maintain a proper horizontal orientation of the transponder coil. The transponder can be attached to the carrier sheet in a number of ways, particularly by affixing the transponder to a central portion of the carrier sheet using a cement or adhesive, as well as mechanical attachment or heat (fusion) bonding.
The above as well as additional objectives, features, and advantages of the present invention will become apparent in the following detailed written description.